History:
The trail was conceived by forester, Benton MacKaye, in 1921. In 1923, the first section of the trail, from Bear Mountain west through Harriman State Park to Arden, New York, was opened. In 1937, the trail was completed to Sugarloaf Mountain in Maine.
See wikipedia.

Length: 2,181 in 2010. The distance and route changes from year to year.
There are over 90 miles of elevation gain.
See AT Elevation Gain and Loss - WhiteBlaze.net
The section that runs just below the summit of Clingmans Dome in Great Smoky Mountains National Park is along the North Carolina and Tennessee border and is the highest point on the trail at 6,643 feet.

Hikers: There are Day Hikers, Weekend Hikers, Section Hikers (They spend one or two weeks on the trail each year) and Thru hikers who do the whole thing in one year.
Thru hikers tend to be individuals who have trail names and get to know each other as they share shelters and communicate with each other via the log books in each shelter.
Most AT hikers are males between the ages of 18 and 29, but we met a man celebrating his 70th birthday on the trail and about 1/4 of the hikers were women.
Famous thru hikers include Supreme Court Justice William O. Douglas.

Thru Hikers:
From 1,500-2,000 people start to thru hike each year (About 20 percent of hikers who begin the trail will complete it.). See number completing the AT by year.

Average Time: 5 - 7 Months.

Speed: Most hikers average 12 to 18 miles per day. A ultra-light (20 lb. pack) hiker I met on the train said she had done a 52 mile day in 15 hrs.

The overall record up to 2011 was set by Andrew Thompson in 2005: 47 days 13 hours 31 minutes. (46 miles/day)
In 2011 Jennifer Pharr Davis set the overall record of 46 days, 11 hours, besting Andrew Thompson record of 47 days in 2005.
Pharr Davis previously set the women's record of 57 days in 2008.
She hikes an 15-18 hrs./day 5 AM to 10 PM averaging almost 3 MPH.
See Asheville woman sets Appalachian Trail record

Shelters: Shelters (lean-to huts) are spaced about a day's hike apart, near water sources. There are usually tent sights around the shelters and a latrine (outhouse).Other accommodations: Most hikers take a day or two off every couple of weeks to get a shower and bed at a local hostel, guest house or motel and a good meal at a restaurant do laundry and re-supply. Some take a break for sightseeing at D.C., New York or other locations.
See Appalachian Trail hostel accommodationsand location information page

Supplies: The trail crosses a road an average of every four miles, making it pretty easy to access trail towns along the way.Many thru-hikers set up a series of maildrops, sending packages to themselves in predetermined towns along the route. These packages, on average, contain about a week's worth of food and supplies to last until the next town where one can resupply.

Trail Angels: Local residents, called Trail Angels, frequently leave coolers with cold water, fruit, ... in coolers where the trail crosses a road.

Ownership:
The trail used to traverse many hundreds of miles of private property; currently 99% of the trail is on public land.